Disclosed herein is an electronic test probe incorporating a unique compression spring construction disposed within a housing and in engagement with a plunger. The invention disclosed herein is an unobvious constructional improvement over applicants' previous inventions disclosed in U.S. Pat. No. 7,315,176 issued Jan. 1, 2008. Furthermore, since the background, the manner of use, the procedures for testing integrated circuits and the like are the same as in applicants' present invention and that of the '176 patent, the disclosure of U.S. Pat. No. 7,315,176 is herewith fully incorporated into the present specification by specific reference thereto.
The present invention utilizes a compression spring that has one longitudinal centerline that is concentric to the housing of the test probe and terminates in a limited number of gradually reduced diameter coil windings that are angled off to one side in a non-concentric manner. The end coil winding of the compression spring engages the conical biasing post on the second end of the plunger that is concentric to the housing, and in this manner the plunger creates an optimal side load, that is, a lateral force, to the plunger contained within the housing to insure full electrical contact between the spring and the probe.
A spring-loaded test probe incorporates an actuating plunger with a first end that makes contact with the device under test (DUT) and a second end that is contained within the housing and engages a compression spring that is also contained within the housing. The geometry of the first end of the plunger is based on the nature of the test target on the DUT. The plunger end might have a sharp conical shape, a spherical shape or multiple sharp points. The geometry of the second end of the plunger is based on the desired performance of the test probe.
When a test probe is used in a fixture to test a device, it is desirable to keep the resistance of the test probe as low and consistent as possible. To ensure low and consistent resistance, the metal components inside a test probe must be precisely located in close proximity to each other, and the normal (lateral) force between the components should be high enough to keep the components in direct contact with each other and to break through any surface barrier resistance that might be in place between the components.
This is often accomplished by putting a side load onto the second end of the plunger. Because of the shape of the plunger's second end, some of the longitudinal force of the compression spring can be redirected onto the plunger perpendicularly, that is, laterally, to the housing's centerline. Putting a side load on the plunger increases the normal force of the plunger on the inside surface of the housing.
In previous implementations, test probes have either used a bias cut plunger in conjunction with a ball bearing or a bias cut plunger without a ball bearing and springs with multiple parallel centerlines (see U.S. Pat. No. 3,416,125 to Cooney and U.S. Pat. No. 7,315,176 to Nelson et al). The compression springs are usually constructed with a single centerline that is concentric to the barrel. The diameter of the ends of the springs might be reduced on the same centerline. An alternative spring design described in applicants' aforementioned U.S. Pat. No. 7,315,176 has a spring with multiple parallel centerlines, but the end coils are concentric with the last section of the compression spring coils.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.